In modern communication systems, the modulation and demodulation of signals often require mixing with other signals at known frequencies. For example, in a cellular phone system, during its communication process, radio frequency (RF) signals of predetermined frequencies are mixed with the signal to be transmitted or received. The local oscillators (LO) that generate the mixer frequencies are commonly implemented by frequency synthesizers.
A frequency synthesizer needs to be configured to generate output at a desired frequency. Typically, programming a frequency synthesizer requires an external processor, such as a microcontroller, an embedded microprocessor, a baseband modem, or a similar device to compute the synthesizer configuration parameters, and then write the parameters over a bus (such as a serial bus) to the synthesizer's register bank. These data words are parameters needed by the synthesizer to configure the desired output frequency. Because of bus speed limitations, the time spent by the microcontroller to perform multiple register writes can be relatively lengthy and the power consumed during each configuration cycle can be significant.
The long initialization time and significant power consumption are particularly undesirable when the synthesizer needs to frequently change the frequency it is generating, such as when the synthesizer is operating in a frequency hopping mode. Many wireless standards support frequency hopping as a way to increase capacity, where a device that functions as a base station or an access point can transmit and receive signals with corresponding carrier frequencies that periodically hop from one channel to the next. The wireless device that communicates with the base station needs to generate one RF signal that matches one carrier frequency for demodulation, and another RF signal that matches the other carrier frequency for modulation. The synthesizers that implement the mobile device's RF LOs need to be reconfigured to generate a new frequency every time the carrier frequency hops. There are other scenarios such as handoff and standby where the synthesizer needs to be reconfigured either due to frequency change or because the synthesizer is powered off and then on. It is desirable to improve upon the current synthesizer implementation to reduce configuration overhead, achieving shorter setup time and lower power.